1. Field of the Invention
This invention is directed to a servo valve having a rotary valving mechanism characterized by low inertia and balanced forces.
2. Discussion of Related Art
Servo valves are used for throttling and directional control of fluidly-driven actuating devices such as linear actuators, seismic chirp-signal generators, motors and the like. The fluid may be pneumatic or hydraulic. The valves may be single-stage or multi-stage, depending upon the load requirements.
The function of a servo valve is to meter fluid flow from a pressure source, P, through hydraulic circuitry to a device to be driven, and thence to a sink (tank) via a return line, R. The driven device has input/output ports, A and B. Depending upon the desired direction of device operation, the servo valve selectively connects a first one of the two intake ports to P and the other port to R and then it connects the other intake port to P and the first one to R. In either position, a metering member in the valve meters the flow of hydraulic fluid at a desired flow rate in a selected direction through the input/output lines to control the direction and operational speed of the fluidly-actuated device. Such valves are known to the art as four-way valves.
For certain applications, such as for a hydraulic jack, a three-way valve is used to apply hydraulic power through line P against a piston in a closed cylinder. Ram retraction is done by causing the valve to open the cylinder to return line R, using gravity or spring loading, rather than hydraulic pressure, as the ram restoring force.
Unless otherwise stated, for convenience and by way of example but not by way of limitation, this disclosure will be explained in terms of a four-way hydraulic servo valve.
As is well known in the hydraulic art, the fluid-metering control member may be a linear spool valve or a rotary valve. In high-speed applications such as encountered with seismic swept-frequency vibrators, the actuator frequency may vary from 5 to 150 Hz over eight or ten seconds. But because of the inherent mass of the spool, the inertia of the spool creates undesirable delays and signal distortions. Further, the fluid flow rate of a spool valve may be inadequate in high-volume applications.
Rotary servo valves have been developed to attempt to ease the problem of inertia. One such valve is described in U.S. Pat. No. 5,467,800, issued Nov. 21, 1995 to John J. Sallas and assigned to the assignee of this invention. The rotary servo valve includes a valve body having a longitudinal bore therethrough, into which a sleeve is inserted. The wall of the sleeve is perforated by two groups of port openings, each group includes a plurality of sets of radially disposed ports. In each group, the central longitudinal axis of one set of ports is radially displaced from the central longitudinal axis of the other set of ports by a preselected angular displacement. At least a third set of ports in one group is in continuous fluid communication with a source of pressurized fluid; at least a third set of ports in the other group is in continuous fluid communication with a return sump. A hollow rotary control member consists of two internal chambers. Each chamber includes a number of sets of apertures that are radially disposed around the walls of the chambers, spaced-apart by a preselected angular separation. A torque motor means is furnished to rotate the control member between two opposite angular positions with respect to a null position thereby to apply power to cause a hydraulic actuator to operate in a desired manner.
The disadvantage of that valve and of others of similar design is the requirement for drilling an axial fluid passageway through the rotor. The diameter of the rotor must be large enough to contain the required cross-sectional area of the fluid conduit. But that is the very thing that must be avoided because the inertia of a tubular or cylindrical rotor increases as the 4th power of the diameter. The mass of the rotor metal on the periphery of a hollow rotor contributes more inertia than the same metallic mass of a solid rotor having a lesser diameter. Thus to reduce the inertia, the peripheral mass must be reduced along with the diameter of the rotor.
A servo valve having a solid rotor is taught by U.S. Pat. No. 4,800,924, issued Jan. 31, 1989 to D. D. Johnson. An electrical-force motor and servo-valve combination has a rotary spool directly coupled to the motor shaft without requiring a rotary-to-linear motion converter; the rotary spool is configured to have a cruciform metering section that cooperates with a flow sleeve having ports and slots configured to produce four-way flow patterns in a reduced length and volume valve package.
The '924 valve would be suitable for low flow applications at best. Further, its design offers severe manufacturing problems.
There is a need for a mechanically robust servo valve characterized by a high flow rate, by a low-inertia metering member or valve gate that is pressure-equalized to prevent structural distortion, and by ease of manufacture.